The present invention concerns a method for manufacturing a drip irrigation tube.
As is well known in the art, tubes of this type are perforated at regular distances with small holes through which the water can flow into the earth at a low flow rate. This flow rate is fixed by flow limiters called "dripper units" mounted within the tube facing the holes and having the shape of a small block attached to the inner surface of the tube. Limitation of the flow rate is assured by a labyrinth formed in the block. A description of such a "dripper unit" can be found in European Patent Application No 0 715 926 filed by the Applicant of the present invention.
A method for manufacturing drip irrigation tubes is known from U.S. Pat. No. 5,324,379. It consists of drawing the drips inside the tube while the latter is being formed in an extruder or an extruding station, the dripper units being attached to the inner wall of the tube via heat welding, when the latter is still warm at the extruder output. In order to do this, the dripper units are introduced into the tube in formation by being drawn by a thread to which they are attached longitudinally at points separated from each other by a distance equal to the distance which will separate them in the completed tube. The aforecited U.S. Patent provides several alternatives for fixing the dripper units to the thread. One of these alternatives, shown in FIGS. 9 to 14 of the Patent, consists in making knots in the thread and fitting each dripper unit with a slit driving lug extending at right angles to the dripper unit's direction of movement in the installation. The knots in the thread can be locked behind these lugs, in order for a given dripper unit to be able to be driven by the preceding one which is already attached to the tube in the extruder die.
The thread is knotted in a knotting station which takes an end of the thread to form each knot, this station being situated upstream of a station distributing the dripper units.
This method has a serious drawback which lies in the fact that it can only be intermittent, i.e. the thread must be stopped each time in order to form the knots. This has a direct effect on the global manufacturing speed of the tube. The other alternatives disclosed in this U.S. Patent allowing the dripper units to be attached to the thread all have the same drawback, the author of the Patent even envisaging preparing threads fitted with dripper units in advance and winding them onto reserve spools. In addition to the risk of seeing the thread and the dripper units becoming entangled when the spool is unwound, the manufacturing process has to be stopped periodically to replace an empty spool with a full spool loaded with a thread and dripper units, an operation which is no more satisfactory than those involved in the other alternatives disclosed.